Method and apparatus for forming a uniform surface potential on photosensitive member

ABSTRACT

A dielectric member is charged by rectifying part of either the positive or negative current output of a corona discharge device by applying the rectified output current to at least one corona wire mounted in spaced relationship with respect to the dielectric member. The positive or negative current is varied so that the effective current in the dielectric member is substantially zero whereby the surface potential of the dielectric member is made uniform. The means for controlling the output current of the corona discharge device is a parallel connection of a variable resistance and a diode.

United States Patent [191 Tanaka Mar. 26, 1974 METHOD AND APPARATUS FOR FORMING 3,076,092 1/1963 Mott 250/495 3,515,548 6/1970 Lange 250/495 x A UNIFORM SURFACE POTENTIAL ON PHOTOSENSITIVE MEMBER [76] Inventor: Susumu Tanaka, 150-20,

Fikaihatayama-cho, Sakai, Japan [22] Filed: Sept. 29, 1971 21 Appl. No.: 184,867

Foreign Application Priority Data Oct. 2, 1970 Japan 45-86860 References Cited UNITED STATES PATENTS 7/1965 Rosenthal ..250/49.5 3/1959 Walkup ..250/49.5

Primary ExaminerWilliam F. Lindquist Attorney, Agent, or Firm-Watson, Cole, Grindle &

Watson ABSTRACT A dielectric member is charged by rectifying part of either the positive or negative current output of a corona discharge device by applying the rectified output current to at least one corona wire mounted in spaced relationship with respect to the dielectric member. The positive or negative current is varied so that the effective current in the dielectric member is substantially zero whereby the surface potential of the dielectric member is made uniform. The means for controlling the output current of the corona discharge device is a parallel connection of a variable resistance and a diode.

3 Claims, 12 Drawing Figures zAIENTEDMARZS m4 3.800.154

sum 2 {1F 3 FIG. 4

V (v) Q 500 Fl G. 5 V M 0 5 90W) 500 g I00 Skv (sec) PAIENTEDmasmn- 8,800,154

SHEET 3 BF 3 FIG. 78

W 0 IE METHOD AND APPARATUS FOR FORMING A UNIFORM SURFACE POTENTIAL ON PHOTOSENSITIVE MEMBER BACKGROUND OF THE INVENTION The present invention relates to both a method and apparatus for charging a photosensitive body or insulator in ele'ctrophotographic apparatus utilizing the characteristic of alternating corona discharge, and more particularly to such a method and apparatus which comprises controlling either positive or negative current in the alternating current corona discharge, controlling the charge of a photosensitive plate by utilizing the automatic balance of the surface potential of the photosensitive body or insulator with the corona current, and adjusting the surface potential.

In general, in electropholographic apparatus, in order to obtain a satisfactory photograph on a photoesensitive plate, for example an electro fax paper, it is necessary to charge uniformly the surface of the photosensitive plate before making an exposure, and further, in order to obtain a uniform photograph and to effect half tone reproduction of an original picture in a satisfactory manner, it is necessary to bring the surface potential of the photosensitive plate to a desired value.

On the contrary, in known techniques for charging a photosensitive body or insulator, direct current corona discharge is usually used. But since that involves unstable corona discharge phenomena, the electric charge of a photosensitive plate is unstable and not uniform.

The above nonuniformity results from:

I. a flaw caused by the nonuniformity of the corona wire in the corona discharge device and the nonuniformity of the emission of corona ions caused by the corona wire being covered with dust;

2. the dielectric breakdown of the photosensitive layer of a photosensitive plate or the dielectric breakdown of the air in the surface of a photosensitive layer caused by the overcharge on the photosensitive plate where an ultrahigh corona voltage or ultrahigh corona are used, especially where the nonuniformity of the density of the surface charge of the photosensitive plate resulting from the ionization on the surface of the photosensitive layer or the same effect in a photosensitive layer by the local electric field which tends to form a small high-voltage point because of the unhomogeneous construction of the photosensitive layer and the dispersing of photoconductive powder in a combined resin as occurs in electro fax paper.

Because the nonuniformity in (1) above cannot be avoided from the practical point of view, in order to lessen its effect ordinarily the corona voltage is increased or the charging time is extended. Also, it is known that steps may be also taken to reduce the effects thereof, by increasing the number of the corona wires, enlarging the emission angle of the ions from the wire of the'stabilizing plate, or coating the wire or vibrating the wire at a right angle to the direction in which the photosensitive plate is transported. But by means of such steps it is impossible to remove the completely nonuniformity.

As regards to the nonuniformity in (2) above, it is necessary to decrease the corona voltage as contrasted with the case in (l) or to recuce the corona charging time. Therefore, the steps required in case (2) are in contradistinction to those in case l and it is therefore difficult to find a satisfactory compromise. As regards the steps to prevent the occurrence of the nonuniformity in (2), the following are known means: to provide a stabilizing plate between the corona wires, to reduce the charging speed by contracting the opening of the corona discharge device, and to increase the space between the wire and the photosensitive plate, etc. But from these steps satisfactory results cannot be obtained.

OBJECTS AND SUMMARY OF THE INVENTION One object of the present invention is to remove the above faults in the prior art, thereby providing charging method and apparatus for placing a uniform charge on a photosensitive body or insulator by means of alternating corona discharge.

Another object of the present invention is to provide such a method and apparatus for arresting either the positive current or negative current of the corona alternating current thereby making it possible to adjust the surface potential of the photosensitive plate.

Still another object of the present invention is to provide such a method and apparatus for arresting part of either the positive current or negative current in the corona alternating current, controlling the arrested current and thereby controlling the charge on the photosensitive plate.

Other and further objects of the present invention are apparent from the accompanying description of the embodiments of the present invention.

The present invention employs a corona discharge device connected to a high-voltage alternating power source, and by means of a controlling member controls either the positive current or negative current of the corona alternating current for, controlling the charge fromed on the photosensitive plate by utilizing the automatic balancing of the surface potential of the photosensitive body or insulator by the corona current. The surface potential of the photosensitive plate is uniform, thereby making it possible to adjust the surface potential to the desired value.

BRIEF DESCRIPTION OF THE DRAWINGS The relation between the corona voltage and corona current of the alternating corona discharge is shown in FIG. 1 (A) and FIG. 1 (B) illustrates a measuring circuit.

FIG. 2 (A) illustrates three characteristic waveforms of the corona current with the corona voltage constant and with different surface potentials of the electrode plate, FIG. 2 (B) shows the relationship between the corona current ic and the electrode plate potential Vp in the upper set of curves and the relationship between the effective current I and the same potential in the lower set of curves and FIG. 2 (C) is a measuring circuit.

FIG. 3 shows characteristic curves of the corona current with respect to variable corona voltage.

FIG. 4 is a characteristic curve diagram showing the relation between the corona volage and the saturation surface potential of the electrode plate (the potential at the intersection point of the positive corona current and the negative corona current in FIG. 3).

FIG. 5 shows the charging speed as a function of the corona voltage.

FIG. 6 shows the various curves of the negative corona currents as a function of variable corona voltage where only the negative corona current is controlled.

FIG. 7 (A) is a circuit which operates in accordance with the principle on which the present invention is formed, and FIG. 7 (B) is an equivalent circuit diagram of the circuit shown in 7 (A) above.

FIG. 8 is a circuit diagram of one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED V EMBODIMENT As regards the characteristics of the alternating corona discharge of the corona discharge device wherein an alternating power source is used, the corona current of the corona discharge device to which a high-voltge alternating current is applied via a boosting transformer T from the alternating power source AC was measured by a synchroscope S used for the observation of two phenomena as shown in FIG. 1(3). The wave forms are as shown in FIG. 1 (A) and the negative current wave forms are greater than the positive current wave forms. Also, it is known that there is the voltage domain where corona discharge is started which causes the discontinuity of the above current wave forms, as the corona discharge, when it is higher than the critical corona discharge voltage, is started and when it is lower than that voltage, is stopped. However, there is a great differnce in the mobility of the ions formed by corona discharge, based on the difference between the above positive and negative current wave forms, and that the mobility of the negative ion is 30 percent to 40 percent greater than that of the positive ion.

The above fact shows that although alternating corona discharge has been widely adopted as a means of removing electric charge, actually it cannot remove electric charge completely from the electrodeplate P because of the difference between the above positive and negative corona currents, but is has the characteristics by which the electrodepate is charged negatively with alternating current and can therefore be adopted as a charging means.

Next, as shown in FIG. 2 (C), with an AC corona voltage applied to the corona discharge device C, and the electrodeplate P biased at a voltage by the DC power source E, the measured corona current with respect to the different bias potentials of the electrodeplate P is illustrated in FIG. 2 (A). As the electrode plate P bias potential changes to -200V, V and +200V, the positive current becomes smaller, whereas the negative current becomes larger.

From the result of the above measurement the relation between the potential V,, on the electrodeplate P and the corona current i i, is illustrated by the upper graph of FIG. 2 (B), representing a positive current curve i, with a point of intersection in the negative potential region and a negative current curve i Also, the effective current I is as shown in the lower graph of FIG. 2 (B).

From the upper graph it is thought that positive ion increase, when the electrodeplate P is of negative. potential, as a result of being attracted by the negative electric field, and decreases and changes as in the curve i when the electrodeplate is of positive potential, as a result of ion repulsion. In contradistinction thereto to the negative ions are effected as shown by the curve i differently than the positive ions. Also, the reason why the intersection point q of the two curves is on the negative potential side is that, as mentioned above, the mobility of the negative ion is greater than that of the positive ion, and this intersection point q shows that the positive and negative currents balance and those currents do not flow at the potential Vq of the electrodeplate P. The effective current I, as shown in the lower graph of FIG. 2 (B), falls to zero at the surface potential corresponding to the intersection point q. This shows that if there is a dielectric membrane on the electrodeplate P, that membrane is charged to a fixed surface potential Vq by means of alternating corona discharge, but at the voltage Vq the corona current balances automatically and the membrane is not charged further.

' The above are the charging characteritic of the alternating corona discharge in the case where the corona voltage is fixed. In cases where the secondary voltage of the transformer T is is changed in the measuring circuit of FIG. 2 (C), the higher that the corona voltage is, the greater are the gradient of curves of positive and negative corona currents, as shown in curves i i, and i i", of FIG. 3. The intersection point moves successively to the negative potential side as shown by points g, g. If the relation between the potential Vq of the intersection points and corona voltage V is illustrated by a graph, it appears as shown in FIG. 4. Further, if relative to each corona voltage V in this case the charging speed at which the dielectric on the electrodeplate P is charged is illustrated by a graph, it is as shown in FIG. 5. This graph shows that relative to all corona voltages the surface potential of the dielectric is charged in the form of curves as in FIG. 5 until it is charged to the automatically balancing surface potential, and further, that when it is above the automatically balancing surface potential it is saturated, and when it is kept within the limits of its surface potential it is controlled by means of the corona voltage.

In addition, when the control of the surface potential is considered from FIG. 3, it may be shown that the control of the corona voltage results in the control of the corona currents, and if the corona currents are controlled, it is possible to control said the automatically balancing surface potential. Also, it may be known in FIG. 3 that, as regards the control of corona currents, if either positive current or negative current is controlled, it is sufficient and that both of them need not be controlled.

That is, as shown in FIG. 6, if negative current only is controlled and changed as in curves i i,- and i the position of the intersection point can be changed as shown by g, g, and g", and the surface potential can be adjusted, although the curve +i, of positive current remains unchanged. In this case, as regards the control of the corona currents, if one current polarity is controlled, it is enough, and the control of that current can be effected in a simpler mannerthan where both positive current and negative current are controlled.

FIG. 7 (A) indicates the principle structure of the present invention utilizing the above mentioned phenomena, in which a control circuit comprising a parallel connection of a variable resistor V and a rectifier D are connected in the ground circuit of the secondary winding of a boosting transformer T. Connected in series with the control circuit to the other end of the secondary winding is the corona wire of a corona discharge device C. The electrodeplate P is grounded, and if the equivalent resistance of the corona discharge device C and the electrodeplate P is expressed as R the equivalent circuit is as shown in FIG. 7 (B).

As regards the positive half-period of the output voltage generated by the secondary winding of the transformer T, all positive voltages are applied to R namely the corona discharge device C, since the variable resistor V is by-passed by the rectifier D, but as regards the negative half-period of the output voltage,

(output voltage) (Rf/R; V the voltage divided by the equivalent resistence Rfof the corona discharge device C and the variable additional resistor V is applied to the corona discharge device C. Accodingly, the negative corona voltage varies according to the change of the resistance value of the variable resistor V only the negative corona current varies as shown in FIG. 6 and the surface potential of the dielectric material mounted on the electrodeplate P varies as shown in FIG. 4.

In the above explanation, the equivalent resistance R of the corona discharge device C varies according to some conditions, such as the form and size of device C, the space between the device and the electrodeplate P, and applied corona voltage, but since the above equivalent resistance has a fixed value in the case where the above conditions are fixed, it is possible to consider only the equivalent resistance of the device. However, as described previously, since there is a difference in the mobility between positive and negative ions, at the time when the positive voltage is applied to the equivalent resistance R and at the time when negative voltage is applied to the resistance equivalent resistance R is different. In addition, as in the diagram, since a sine wave alternating input is applied to the above equivalent resistance R it is necessary to consider R, as a function of time.

FIG. 8 illustrates an example of the present invention which embodies the principle structure of the present invention as shown in FIG. 7 (A). Corona discharge device C is provided with two corona wires W and W each 200 mm in length, and which are suspended respectively in the center of and within shield frame g and h which are 22 mm in width. Intermediate stabilizing plate f isinterposed between the wires which are suspended at a distance of l 1 mm from the upper part of the frames. Grounded electrodeplate P is mounted at a distance of mm from the wires. One end of the 799 secondary coil of the boosting transformer T is c onnected to the alternating power source A was connected to the above corona wires W, and W and to the other end of the above secondary coil the variable resistance V whose resistance value may be changed up to a maximum of 500 K O, and the commutating element D in parallel with the variable resistance V R are connected and grounded. With dielectric H, which is 5 ,u. thick, and has a dielectric constant of 2.5 is mounted on the electrodeplate P and an alternating corona discharge is effected with the resistance value of the variable resistance V set to zero Q, a saturation surface potential of -30OV is obtained on dielectric H within approximately 2 seconds.

Further, when the resistance values of the variable resistor V were changed in the order of 100 k0, 200

KG, 300 KG, 400 K0, and 500 K!) and the saturation surface potential on the dielectric H was measured, the following values were obtained as in Table I:

TABLE I Variable Resistance Value Surface Potential K P) 0 300 I00 -240 200 -l 300 l30 400 500 50 From the above Table I it has been found that by changing the resistance value of the variable resistance V the saturation surface potential on the dielectric H can be set as desired. The present invention is designed with dielectric H on the electrodeplate P and so as to charge the photosensitive plate while it is stationary while it is moved with respect to the corona discharge device C.

The present invention, as described above, can form a uniform charge onto the photosensitive plate by means of the alternating corona discharge of the corona discharge device using a high-voltage alternating power source. Also the charge is fixed by the automatic balancing characteristics of the alternating corona discharge, and exactly the same surface potential set in advance can be formed without regard to the kind of photosensitive plate used by controlling either the positive current or the negative current of the alternating corona current. Moreover because controlling either the positive current or the negative current as mentioned above is sufficient control the apparatus is very simple, inexpensive and yet durable as compared with such commutating elements as variable resistors, di-

odes, etc. In addition, according to the present invention, with respect to either the positive residual charge or the negative residual charge left on the photosensitive plate it is possible to charge the surface potential uniformly and as desird, and therefore, in cases where a photosensitive plate is used repeatedly as in Xerography, the invention can do without the process of removing charge and any charge removing device. additionally, the invention affords a different gradation of copies by the adjustment of the variable resistance.

What is claimed is:

1. A device for charging a dielectric member in an electrophotographic device comprising:

a high-voltage alternating power source;

a corona discharge device including at least a corona wire connected to one pole of said high-voltage alternating power source;

an electrode plate connectable to the other pole of said high-voltage alternating power source for supporting said dielectric member thereon facing said corona wire with a gap therebetween so as to make a corona discharge circuit through said dielectric member; and

control means including rectifier means and variable resistor means connected in parallel with each other to said other pole whereby said rectifier means provides a DC current of one polarity and said variable resistor means provides a DC balancing current of the opposite polarity through said corona discharge circuit.

2. A device for charging a dielectric member in an electrophotographic device as in claim 1, wherein said high-voltage alternating power source is a boosting transformer having a primary winding connected to an alternating power source, and a secondary winding having said one pole and said other pole; said electrode plate is grounded; and said rectifier means is a diode, said diode and variable resistor being connected in parallel with each other and connected to said other pole and ground.

3. A method for controlling the charging of a dielectric member in an electrophotographic device comprising the steps of:

rectifying either the positive or negative current output of a corona discharge device by rectifying means connected to a high voltage alternating power source;

varying the non-rectified current through variable resistor means connected to said alternating power source in parallel with said rectifying means;

applying the rectified output current and the nonrectified current to said corona discharge device;

mounting said dielectric member on an electrode plate facing said corona discharge device with a gap therebetween so as to form a corona discharge circuit through said dielectric member; and

controlling the charge formed on said dielectric member to provide a uniform surface potential on said dielectric member by varying the non-rectified current to said corona discharge device by varying the resistance of said variable resistor means so that the effective current in said dielectric member is substantially zero.

' UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 154 Dated March 26, 1974 Inventor) Susumu Tanaka It is certified thatv error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

[ 73] Assignee: MINOLTA CAMERA KABUSHIKI KAISHA Osaka-shi, Osaka-fu, Japan Signed and sealed this 17th day of September 1974.

(SEAL) Attest:

MeCOY My GIBSON JR. C. MARSHALL DANN Commissioner of Patents Attesting Officer FORM po'mso (1069) USCOMM-DC wave-ps9 U S GOVERNMENT PRINTING OFFICE p59 O-366-334, 

1. A device for charging a dielectric member in an electrophotographic device comprising: a high-voltage alternating power source; a corona discharge device including at least a corona wire connected to one pole of said high-voltage alternating power source; an electrode plate connectable to the other pole of said highvoltage alternating power source for supporting said dielectric member thereon facing said corona wire with a gap therebetween so as to make a corona discharge circuit through said dielectric member; and control means including rectifier means and variable resistor means connected in parallel with each other to said other pole whereby said rectifier means provides a DC current of one polarity and said variable resistor means provides a DC balancing current of the opposite polarity through said corona discharge circuit.
 2. A device for charging a dielectric member in an electrophotographic device as in claim 1, wherein said high-voltage alternating power source is a boosting transformer having a primary winding connected to an alternating power source, and a secondary winding having said one pole and said other pole; said electrode plate is grounded; and said rectifier means is a diode, said diode and variable resistor being connected in parallel with each other and connected to said other pole and ground.
 3. A method for controlling the charging of a dielectric member in an electrophotographic device comprising the steps of: rectifying either the positive or negative current output of a corona discharge device by rectifying means connected to a high voltage alternating power source; varying the non-rectified current through variable resistor means connected to said alternating power source in parallel with said rectifying means; applying the rectified output current and the non-rectified current to said corona discharge device; mounting said dielectric member on an electrode plate facing said corona discharge device with a gap therebetween so as to form a corona discharge circuit through said dielectric member; and controlling the charge formed on said dielectric member to provide a uniform surface potential on said dielectric member by varying the non-rectified current to said corona discharge device by varying the resistance of said variable resistor means so that the effective current in said dielectric member is substantially zero. 